1. Field of the Invention
The present invention is directed to bucket elevators which are generally constituted by a drive member, such as an elastomer-based flexible belt, provided with a plurality of buckets on one of its faces and travelling continuously round a bottom pulley situated at the base of elevator and a top pulley, with the drive member thus having a rising length and a falling length.
2. Discussion of Background and Material Information
In conventional bucket elevator systems, the buckets are loaded by inserting the material to be elevated near the base of the elevator, close to the bottom pulley either on the rising side or on the falling side. In this respect it should be mentioned that the bucket belt in enclosed in casing which comprises one or two legs surrounding the said rising and falling lengths, a bottom box in which bucket loading takes place, and a top box for receiving and removing the material from the buckets as they pass over the top pulley. The bucket are loaded by means of an inclined chute (or by two chutes, one on the rising side and the other on the falling side) with the material to be elevated being poured into the bottom box via the chutes at a rate which depends on elevator throughput.
This method of loading the elevator has numerous drawbacks, and hitherto it has been necessary to accept such drawbacks since it has not been possible to control the dynamic phenomena which take place in the casing and which degrade the material, wear the elevator, produce dust which may give rise to explosions, consume excessive power, and limit the maximum speed of the belt and hence the maximum throughput of the elevator.
The above-mentioned phenomena have multiple causes which are somewhat different depending on whether the buckets are loaded on the rising side or on the falling side of the belt.
When loading takes place on the rising side, the material being loaded (a granular or powder material) is admitted into the bottom box travelling in the opposite direction to the buckets. The resulting shocks damage friable materials, and if the material to be transported is abrasive, they also give rise to rapid bucket wear. Additionally, fine break-away particles constituted the said dust, (a possible cause of explosions), which dust is driven upwardly and may permeate an entire installation (storage silos, bins, . . . ) of which the elevator constitutes a part. One way of reducing these drawbacks consists in limiting the speed of the buckets and consequently in reducing the maximum throughput of the elevator.
Loading the buckets on the falling side of the belt has therefore been considered, since on this side the material being loaded into a bucket is moving in the same direction as the bucket. This reduces the violence of the resulting shocks, but it has been observed that full buckets passing round the bottom pulley tend to eject some of the material with which they are loaded by virtue of centrifugal force. As a result a pile of material builds up at the bottom of the elevator adjacent to the rising side thereof and this pile is constantly topped up by the material spilling out from the buckets. The buckets are dragged through this pile, and in particular their outermost portions (which have the highest linear velocity since they are furthest from the center of the pulley) crush the material in the pile, thereby abrading the buckets and making dust. Furthermore, this pile is stirred up by the buckets passing through it and the particles of material in the pile rub against one another and against the walls of the bottom box, which is rapidly degraded. It should be observed that this recycling phenomenon and the presence of a pile of material to be elevated at the bottom of the elevator is also observed when bucket loading takes place on the rising side, since some of the material to be elevated always spills or bounces out from the buckets.
Thus both types of bucket loading have numerous drawbacks which, hitherto, it has only been possible to mitigate the drawbacks by limiting the speed of the elevator, and thus its throughput, or by providing dust-removing devices or anti-explosion devices such as ejectable flaps in the elevator boxes.
Preferred embodiments of the present invention provide an elevator whose buckets can be loaded while avoiding the above-mentioned drawbacks. This makes it possible to drive the elevator belt at a speed which is at least equal to the critical speed as conventionally measured at the outermost point of the buckets as they pass round the bottom pulley. It also makes it possible to handle extremely fragile materials using standard equipment. Furthermore, the invention makes it possible to obtain high throughputs using compact and relatively cheap elevators.